RFID technology generally involves interrogating an RFID tag with radio frequency (RF) waves and reading the responding RF waves with an RFID reader. An RFID tag typically includes a miniscule microchip coupled to an RF antenna. RFID tags can be attached to the object to be identified. An RFID reader typically includes an antenna coupled to a transmitter and a receiver.
In many implementations, the antenna for the RFID tag has a null-direction in which no RF wave is radiated. For example, the radiation from a dipole antenna follows a toroidal radiation pattern with the central axis of the toroid representing the null-direction of the dipole antenna. When an RFID tag is located in the radiation range of the RFID reader, the electrical filed of the RF waves is generally transverse to the direction of propagation. The RFID tag tends to responds most to interrogating RF waves from the RFID reader when the null-direction of the antenna for the RFID tag is also transverse to the propagation direction of the interrogating RF waves. For example, as shown in FIG. 1A, interrogating RF waves with the x-polarization are generated by the RFID reader 20, and the RFID tag 60 tends to response most when the null-direction 65 of the antenna is also aligned in the x-direction. In another example, as shown in FIG. 1B, interrogating RF waves with the circular polarization are generated by the RFID reader 20, and the RFID tag 60 tends to response most when the null-direction 65 of the antenna is transverse to the propagation direction of the interrogating RF waves.
The RFID tag tends not to response to the interrogating RF waves from the RFID reader when the null-direction of the antenna for the RFID tag is aligned in the propagation direction of the interrogating RF waves. For example, as shown in FIGS. 2A-2B, the RFID tag 60 tends not to response to the interrogating RF waves when the null-direction 65 of its antenna is aligned in the z-direction, the propagation direction. Consequently, it can be difficult to read some of the RFID tags placed in front of an RFID reader when these RFID tags are incorrectly aligned with RFID reader. There is a need for a method and apparatus for improving the reading of RFID tags with RFID readers.